Hey, everyone. Welcome back to topic number two in our database class. In this video, part four, I'm going to discuss composite keys. The idea with a composite key is that we want to have a unique key, but we want to get that uniqueness through the combination of two or more values. Now, this is something that I feel is best understood through an example. So let's take a look at a flight example. Okay, most people, most of us, I hope have been on at least one airplane flight in our lives. So this I hope will be a reasonably familiar context for us. But as we know, the way that airlines operate their business is they use flight numbers. So you might, for example, be on, I don't know, British Airways flight number 33 or say, want us airlines flight number 12 or United Airlines flight 74, whatever it may be. So this is how these airlines uniquely identify a flight. Now, what a flight number is, for the perspective of an airline is a trip on an airplane from one location to another location on a specific day. So let's imagine that I'll try to draw the United States here a little bit. Here's my continental, you're not so bad. Yes, this is why I teach information systems rather than art. Now, let's imagine that we have a flight. That originates, I don't know, say in Orange County. So we're, that's probably not a very good Orange County. We're probably down here a little more about like there. All right, so let's say that we're there and we want to go to, I don't know, say New York City. And so we're going to be here somewhere. Now, if we're on, let's say United Airlines and we want to fly from here in Orange County to New York City at, I don't know, eight 30 AM, that is going to be a particular flight. So let's say that is United Airlines flight number 33. So United Airlines flight number 33 is this flight here. Right. It's this eight 30 AM flight from Orange County to say New York City. And so I'll just put that up there. He 30 AM. Orange County. New York City. And every other flight will have the same sort of meaning. Right. So if I have United Airlines flight 42. Instead of 33 maybe 42 is a flight from, I don't know, like Seattle to Orlando. Right. And so I'm going to go to the airport from San Francisco to Dallas. Whatever that is, it's going to have unique flight. But what it means is it's this particular airline, making this particular journey at this time of day. So what that means then is we're only going to have one United Airlines flight 33 per day, because it's the eight 30 AM flight from Orange County to New York. And so we're going to have British Airways flight 12, which might be the two PM flight from New York to London. But there's only going to be one of them per day. So there's only one United Airlines flight 33 per day. There's only one British Airways flight 12 per day. And so on. And so we're having all these concepts in mind. We can see that we could not possibly use. The flight number alone as a way of uniquely identifying a specific flight. And so if I have United Airlines. I know that's the eight 30 AM flight from Orange County to New York, but this can occur on different days. So if I say today is the seventh of September. So I'll just write this as the seventh of September. So I know that flight took place at eight 30 AM on September 7th from Orange County to New York. I might have another flight say British Airways flight number 12. And let's say that also occurred on the seventh of September. This is our two PM flight from New York to London. That's British Airways flight 12 in our little world here. I know that these flights repeat on a daily basis. So I can have another United Airlines flight 33 in here. Which is a duplicate of course of the one that we have previously. But maybe this is yesterday's flight. So this was yesterday's flight at eight 30 AM. On United Airlines from Orange County to New York. So here's the idea with a composite key. If I have some data like these, imagine this is our flight number column. And this is our date column. So if I have data like these, we can see that within the flight number column. We have repeating values. So I have two United Airlines flight 33 is in here. Sorry my table is a little weird looking. But the idea. So this value is duplicated. There's two of them. So clearly I cannot use this column by itself. As a basis for uniquely identifying a row within the table. All right, because there are repeating values of flight numbers. We have one for every day. Similarly, if I look at this column, which is a date column, we can see that we have repeating values here as well. All right, so these two, for example, are flights that both took place on the seventh of September. Therefore, we cannot use this column by itself as a way of uniquely identifying a row. Either, right, because this has duplicate values. So this column cannot be a unique key. This column cannot be a unique key. However, the idea with a composite key is that the combination of these values is unique. So within this table, there is only one United Airlines flight 33 that took place on September 7. There's another United Airlines flight 33 here, but that took place on a different day. So United Airlines 33 on September 7. Here's another flight on September 7, but that was not United Airlines 33. That one was British Airways flight number 12. So the idea with a composite key is that within the table, the combination of these values is unique. If there's no other row in this table that has United Airlines flight 33 on September 7. There's no other row in this table that has British Airways flight 12 on September 7. Or United Airlines flight 33 on September 6. So the combination of the values delivers the uniqueness. This is the idea of a composite key. Individually, the values within a column are not unique. But the combination of values from two or more columns. Guarantees uniqueness. So in this world of airline flights, it's not enough to know the flight number by itself. It's not enough to know the date by itself. If we want to be able to uniquely identify a specific flight. That is with a specific real world airplane with pilots and crew and customers and baggage, leaving from say Orange County and flying to New York on a specific date and time. We could identify that unique flight through a combination of its flight number and date, but not by using either of those pieces of information individually. Both of them to get the uniqueness.